The spine is a flexible column formed of a plurality of bones called vertebrae. The vertebrae are hollow and piled one upon the other, forming a strong hollow column for support of the cranium and trunk. The hollow core of the spine houses and protects the nerves of the spinal cord. The different vertebrae are connected to one another by means of articular processes and intervertebral, fibrocartilaginous bodies. Various spinal disorders may cause the spine to become misaligned, curved, and/or twisted or result in fractured and/or compressed vertebrae. It is often necessary to surgically correct these spinal disorders.
The spine includes seven cervical (neck) vertebrae, twelve thoracic (chest) vertebrae, five lumbar (lower back) vertebrae, and the fused vertebrae in the sacrum and coccyx that help to form the hip region. While the shapes of individual vertebrae differ among these regions, each is essentially a short hollow shaft containing the bundle of nerves known as the spinal cord. Individual nerves, such as those carrying messages to the arms or legs, enter and exit the spinal cord through gaps between vertebrae.
The spinal disks act as shock absorbers, cushioning the spine, and preventing individual bones from contacting each other. Disks also help to hold the vertebrae together. The weight of the upper body is transferred through the spine to the hips and the legs. The spine is held upright through the work of the back muscles, which are attached to the vertebrae. While the normal spine has no side-to-side curve, it does have a series of front-to-back curves, giving it a gentle “S” shape. If the proper shaping and/or curvature are not present due to scoliosis, neuromuscular disease, cerebral palsy, or other disorder, it may be necessary to straighten or adjust the spine into a proper curvature.
In some instances, a vertebral body may become severely damaged due to tumor or trauma. The vertebral body may be removed along with the adjacent intervertebral discs to on both sides. A cage typically may be put in place in the region formerly occupied by the vertebral body and the adjacent intervertebral discs. The cage must be secured between the adjacent superior and inferior vertebral bodies by compression and supplemental fixation to promote successful bone fusion.
One approach to secure the cage between the vertebral bodies may include a rigid system of screws and rods on a posterior aspect of the spine. The screws may be inserted into the pedicles of the vertebrae where the bone includes mostly cortical material to act as bone anchors, and the rods may be inserted into heads of the screws. Two rods may run substantially parallel to the spine and secure the spine in the desired shape and curvature. Instruments may be used to compress the attached vertebrae together to tightly grip the cage between them. Another approach to secure the cage between the vertebral bodies includes screws and rods on a lateral or anterior aspect of the spine. Yet another approach includes a Kaneda-style lateral plate.